The present invention relates to a beam shaping optical system that affects a cross-sectional shape of a light beam. In particular, the present invention relates to a beam shaping optical system used in a light emitting device or an imaging optical system.
U.S. Pat. No. 4,948,223 (hereinafter xe2x80x9cPatentxe2x80x9d) discloses a light shaping optical system as shown in FIG. 7. The optical system includes first and second wedge prisms 1 and 2 that are arranged in order from an incident side (i.e., a side from which a light ray enters the optical system, the left side in FIG. 7). The beam shaping optical system operates to, for example, convert an elliptical-shaped light beam (for example, as emitted from a semiconductor laser) into a circular-shaped light beam. The prisms 1 and 2 have principal sections that are parallel to each other, where a principal section of a wedge prism is defined as a plane perpendicular to both of the two refractive surfaces of the wedge prism through which a light beam passes. In other words, the principal section is defined as a plane perpendicular to a ridgeline of the refractive surfaces.
This type of beam shaping optical system is generally included in an optical system of an optical disk device, a magneto-optic disk device, a laser beam printer, or the like in order to shape a cross-sectional shape of a light beam emitted by a light source such as a semiconductor laser so that the light beam can be used effectively, or in order to form a symmetrical beam spot on an object surface.
Further, in order to increase the speed of such devices (laser beam printers and the like) a plurality of light beams may be simultaneously used to read or write information. In this case, a multi-beam semiconductor laser (not shown) can be used as a light source. A multi-beam semiconductor laser includes more than three light emitting points arranged in a single element and principal rays from the light emitting points forms predetermined angles with one another.
However, since the conventional beam shaping optical system disclosed in the Patent is designed for converting the cross-sectional shape of a single light beam, there is a problem in that, if there is a minor variation in an incident angle, due to difference in exit angles of light beams emitted from light emitting points of a multi-beam semiconductor laser, there is a relatively large change in angular magnification by the beam shaping optical system. The following TABLE 1 is based on data disclosed in the Patent and shows a relationship between the incident angle xcfx86 (deg.) to the first prism 1 and the exit angle xcfx89 (deg.) from the second prism 2 to illustrate the occurrence of an exit angle error xcex4 for variations in the incident angle xcfx86. The TABLE 1 also shows an imaginary exit angle "psgr", an angular magnification xcex3, a percentage difference xcex3xe2x80x2, and the exit angle error xcex4. The imaginary exit angle "psgr" is defined as an exit angle when the angular magnification is constant, i.e., the imaginary exit angle xcfx89 is determined by multiplying the incident angle xcfx86 by a paraxial angular magnification xcex30 (equal to 0.669041 in this case). The angular magnification xcex3 is the actual angular magnification and the percentage difference xcex3xe2x80x2 is a percentage difference between the actual angular magnification xcex3 at each incident angle with respect to the paraxial angular magnification xcex30, that is, xcex3xe2x80x2=(xcex30xe2x88x92xcex3)xcex30. The exit angle error xcex4 is obtained by subtracting the imaginary exit angle "psgr" from the exit angle xcfx89.
Thus, the amount of change of the angular magnification with respect to a change in the incident angle is relatively large in the conventional beam shaping optical system. Therefore, when the conventional beam shaping optical system is used for a multi-beam optical system with a multi-beam semiconductor laser, beam spots formed on an object surface are positioned at irregular intervals even if the emitting points of the light beams are positioned at regular intervals.
Further, the exit angle error in the conventional beam shaping optical system cannot be counterbalanced by altering a rotationally symmetric lens such as a collimator lens or an objective lens of the optical system. Because the exit angle error contains distortion that are represented by even order functions of the incident angle (as shown in FIG. 3), whereas distortion of a rotationally symmetric lens such as a collimator lens or an objective lens is represented by third or higher order, odd order functions.
It is therefore an object of the present invention to provide a beam shaping optical system in which an amount of change in angular magnification due to a change in incident angle is reduced. A further object of the present invention is to provide a light emitting device using the beam shaping optical system in an imaging optical system to converge light beams emitted from light emitting points aligned at regular intervals to form beam spots at regular intervals.
According to an aspect of a beam shaping optical system according to the present invention, which is provided with a first wedge prism having two refractive surfaces defining a first principal section, and a second wedge prism having two refractive surfaces defining a second principal section that is parallel to the first principal section, wherein a reference ray of light forms incident angles at each of the refractive surfaces of the prisms satisfying the following condition (1):                               "LeftBracketingBar"                                    ∑                              j                =                1                            L                        ⁢                          α              j                                "RightBracketingBar"                 less than                   0.020          .                                    (        1        )            
where
j is a refractive surface number defined from the incident side,
L is a total number of refractive surfaces, and       α    j    =            ∏              k        =        1                    j        -        1              ⁢                  γ        k            ·              ϵ        j            
where
Π is an operator to give multiplication of all of the elements,             γ      j        =                  cos        ⁢                  xe2x80x83                ⁢                  θ                      0            ⁢            j                                                (                                                    n                                  1                  ⁢                  j                                2                            /                              n                                  0                  ⁢                  j                                2                                      -                                          sin                2                            ⁢                              θ                                  0                  ⁢                  j                                                              )                          1          /          2                                ϵ      j        =          -                                    (                                                            n                                      1                    ⁢                    j                                    2                                /                                  n                                      0                    ⁢                    j                                    2                                            -              1                        )                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ                          0              ⁢              j                                                            (                                                            n                                      1                    ⁢                    j                                    2                                /                                  n                                      0                    ⁢                    j                                    2                                            -                                                sin                  2                                ⁢                                  θ                                      0                    ⁢                    j                                                                        )                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ                          0              ⁢              j                                          
where
xcex80j is an incident angle of said reference light ray to the j-th surface,
n0j is a refractive index of a medium at the incident side of the j-th surface, and
n1j is a refractive index of a medium at the exit side of the j-th surface.
The beam shaping optical system of the present invention may be arranged in a light emitting device that is provided with a light source including a plurality of light emitting points and a collimator lens for converting light beams from the light source into parallel beams. In such a light emitting device, the beam shaping optical system is positioned such that the exit light beams from the collimator lens are made incident thereon.
In another aspect of the invention, a light emitting device comprises a light source having a plurality of light emitting points, a collimator lens for converting the light beams from the light source into parallel beams, and a beam shaping optical system for converting a sectional shape of each of the light beams from the collimator lens. The beam shaping optical system comprising two wedge prisms, and the collimator lens has distortion to compensate distortion generated by the beam shaping optical system.
In a still further aspect of the invention, an imaging optical system comprises a light source having a plurality of light emitting points, a collimator lens for converting the light beams from the light source into parallel beams, a beam shaping optical system for converting a sectional shape of each of the light beams from the collimator lens and an objective lens for converging the light beams from the beam shaping optical system onto a medium. The beam shaping optical system comprising two wedge prisms, and the objective lens has distortion to compensate distortion generated by the beam shaping optical system.